1. Technical Field
The present invention relates to optical network communication systems, and more particularly, to Optical Frequency Division Multiple Access (OFDMA)-Passive Optical Network (PON) communication systems.
2. Description of the Related Art
Next generation optical access networks are expected to provide simultaneous delivery of multiple services for a number of different customers over a common network architecture. For example, such services may include providing legacy Digital Signal 1 (T1)/E1 carrier traffic, digital multimedia transmission, backhaul for cellular base stations, layer-2 Virtual Private Network (VPN), security channels for storage networks, etc. To provide such services while effectively managing costs, service providers should have a certain flexibility and should be capable of supporting heterogeneous optical network units (ONUs) employing a variety of services and data rates at the customer premises of a given optical access network.
Several different optical access network architectures have been developed to address the problem. Time division multiplexing (TDM)-based Gigabit Ethernet (GE)-Passive Optical Network (PON) and 10 Gigabyte (G)-PON network 100, illustrated in FIG. 1, is one exemplary optical access network that is currently known in the art. Network 100 may include an optical network unit (ONU) 104 configured to transmit Ethernet packets 106 to an optical line terminal (OLT) 102 using an analog signal to packets converter 108 and a Hub/Router 110. In turn, the OLT 102 may receive the Ethernet packets 106 and process the packets using a Hub/router 112 and a packets to analog signal converter 114. Here, the OLT 102 apportions usage in a time-domain round-robin fashion among multiple ONUs 104 such that, during each time slot, only one ONU can transmit or receive signals. As a result, TDM-based PONs require complex scheduling algorithms and framing technology to support heterogeneous services. Moreover, performance is highly sensitive to packet latency and the services are not transparent to other traffic concurrently flowing through the same link. Furthermore, in TDM-based PONs, it may be difficult for a single OLT to concurrently support ONUs with different line rates.
Another optical access network system known in the art includes a Wavelength Division Multiplexed (WDM)-PON 200, illustrated in FIG. 2, which assigns each ONU in the network a different wavelength for transmitting and receiving signals. WDM-PON system 200 may include a plurality of ONUs 202-206, an OLT 208 and WDM Multiplexers 210 and 212, which may be implemented with arrayed waveguide gratings. In the particular architecture shown in FIG. 2, ONU 202 may be assigned wavelengths λ1 and λ2 for transmission of analog baseband T1/E1 signals 214 and Ethernet packets 216, respectively, and ONU 202 may employ transmitters 222 and 224 to transmit converted signals 214 and 216 on wavelengths λ1 and λ2, respectively. Similarly, ONU 204 may be assigned wavelength λ3 for transmission of analog wireless signals 218 using transmitter 226 and ONU 206 may be assigned wavelength λ4 for transmission of Ethernet packets 220 using transmitter 228. The multiplexer 212 may be configured to multiplex optical carriers transmitted from the ONUs and multiplexer 210 in OLT 208 may receive, demultiplex and forward the signals to a receiver array in the OLT 208. For example, signals transmitted along wavelengths λ1-λ4 may be respectively forwarded to receivers 230-236 and further processed in the OLT 208 to obtain signals 214-220, respectively.
A WDM-PON architecture can transparently deliver multiple services to a collection of ONUs, as each ONU can use a dedicated wavelength. Colorless WDM-PON architectures are also currently available. However, this multiple wavelength arrangement requires multiple transceivers and arrayed-waveguide gratings or optical filters to correctly distribute wavelengths, which notably increases both system cost and complexity. In addition, WDM-PON lacks the flexibility to dynamically allocate bandwidth resources among different services. Further, while WDM-PON can support heterogeneous ONUs through a receiver array at the OLT, it still lacks the ability to readily and easily upgrade the ONUs at customer sites. For example, because each ONU is wavelength-specific, all ONU upgrades mandate corresponding changes at the OLT.
WDM Orthogonal Frequency Division Multiple Access (OFDMA)-PON is another known optical network architecture. In OFDMA-PON, the time and orthogonal-frequency domain bandwidth resource allocation is controlled by the OLT and communicated to the ONUs over non-reserved Orthogonal Frequency Division Multiplexing (OFDM) subcarriers and pre-configured time slots, such that each ONU can be assigned one or more subcarriers in a given time slot. With reference to FIG. 3, one exemplary OFDMA-PON 300 is illustrated. Network 300 may include an OLT 302, ONUs 304, 306 and 308, which may be respectively located at a business area 310, a mobile station 312 and a residential area 314. ONU 304 transmits Ethernet packets 316 over a wavelength λ1, ONU 306 transmits analog wireless signals 318 over λ2 and ONU 308 transmits Ethernet packets 320 over λ3. Upstream data signals may be transmitted from the ONUs to the OLT by employing a splitter/coupler 322, which multiplexes signals received by different ONUs. The overall bandwidth can be divided into both orthogonal frequency-domain subcarriers and time-domain slots, such that each ONU can be assigned one or more subcarriers in a given time slot. One ONU frame is illustrated in element 324, with frequency as the vertical axis and time as the horizontal axis. Element 326 illustrates the bandwidth allocation to the various ONUs with frequency as the vertical axis and time as the horizontal axis. As noted above, the time and frequency domain bandwidth resource allocation is controlled by the OLT. In this way, the OLT in an OFDMA-PON can support heterogeneous ONUs using a single OLT and single optical receiver. However, for upstream traffic, each ONU would still require the use of different wavelengths to avoid broadband beating noise that would otherwise be generated at the OLT receiver due to mixing of multiple optical carriers from the ONUs. Under this arrangement, the upstream OFDMA-PON would need to be combined with WDM technology, which reduces its cost-efficiency.